Use of a glass shroud in a metal halide arc lamp assembly

Abstract: A double-enveloped lamp assembly (10) includes a light-source capsule (16), which is substantially surrounded by a light-transmissive shroud (14) made of glass, preferably aluminosilicate glass. In a preferred embodiment, the lamp assembly (10) is a metal halide arc discharge lamp, and the shroud (14) is a cylindrical, open-ended tube. For metal halide lamps having a power rating of 150 watts or less, the shroud (14) preferably has a wall thickness of one millimeter. The thin wall glass shroud meets or exceeds containment performance requirements.

Description: The invention relates to the use of a light transmissive glass shroud in accordance with the generic clause of claim 1. Double-enveloped lamps, such as metal halide arc lamps, include a light-source capsule, such as an arc tube, and a sealed outer envelope surrounding the light-source capsule. The arc tube typically operates at high temperature and high pressure. In such lamps, there is a small probability that the light-source capsule will burst. When this occurs, hot fragments of fused quartz from the arc tube are forceably propelled against the outer envelope. If the outer envelope also shatters, there is a potential safety hazard to persons or property in the immediate surroundings. A failure of the outer envelope is known as a containment failure. From document GB-A-20 56760 there is already known the use of a borosilicate glass shroud placed within a sealed light-transmissive outer envelope of a metal halide arc lamp assembly and substantially surrounding a metal halide arc tube for the prevention of the breakage of said outer envelope in the event of an arc tube explosion, wherein said metal halide arc tube is mounted within said outer envelope and wherein said halide are lamp assembly further comprises means for supplying electrical energy to said arc tube. European Patent Application No. 416 705 A1 discloses a high-pressure discharge lamp having an inner sheath of quartz glass around the lamp vessel and an outer sheath of aluminosilicate glass around the inner sheath. Document EP-A-19850 describes the conventional structure of a halogen incandescent lamp, comprising a bulb which is constructed of aluminosilicate glass, having a strain point of 660°C or higher, so that the bulb will hardly suffer any undesirable cracks even if it is heated to a temperature provided for the performance of halogen cycle, i.e. 250°C or higher. European Patent Application No. 181 690 A1 relates to the production of transparent, high silica glass envelopes for tungsten halogen lamps, whereby said glass exhibit annealing points between about 900 to 925°C, while thermal stability and low water content, i.e., a beta OH value of less than 0.4mm -1 , is retained. United States Patent No. 5,122,706, issued June 16, 1992 to Parrott et al., discloses a metal halide arc lamp wherein a generally cylindrical fused quartz shroud encircles an arc tube. Typically, the shroud is made of fused quartz and is at least about 2.0 mm (0.098 inches) thick. The shroud may be open at both ends, or may be open at one end and have a domed configuration at the other end. The shroud is typically mounted within the lamp by clips or straps attached to a frame member. Fused quartz has been used to fabricate prior art shrouds because it is a reliable material which has a very low water content and is resistant to high operating temperatures. Furthermore, the arc tube which the shroud surrounds is usually fused quartz. Shrouds for metal halide arc discharge lamps are also disclosed in U.S. Patent Nos. 5,023,505, issued June 11, 1991 to Ratliff et al.; 4,812,714, issued March 14, 1989 to Keeffe et al.; 4,888,517, issued December 19, 1989 to Keeffe et al.; 4,791,334, issued December 13, 1988 to Keeffe et al.; 4,721,876, issued January 26, 1988 to White et al.; 4,709,184, issued November 24, 1987 to Keeffe et al.; 4,625,141, issued November 25, 1986 to Keeffe et al.; 4,620,125, issued October 28, 1986 to Keeffe et al.; and 4,499,396, issued February 12, 1985 to Fohl et al. It is also known to reinforce a shroud by using a mesh of substantially non-conducting fiber, as disclosed in U.S. Patent No. 4,942,330, issued July 17, 1990 to Karlotski et al. In addition to containment, the shroud has other beneficial effects on lamp operation. In lamps with a gas filled outer envelope, the shroud reduces convective heat losses from the arc tube and thereby improves the luminous output and the color temperature of the lamp. In lamps with an evacuated outer envelope, the shroud helps to equalize the temperature of the arc tube. It would be desirable to replace the fused quartz shroud with a different material which could be easily manufactured, which would provide the benefits of a fused quartz shroud, but which would require less material. Accordingly, it is an object of the present invention to provide an improved shroud for containing an arc tube in the event that the arc tube bursts. It is another object of the present invention to provide a shroud which is lighter in weight than the fused quartz shroud, but which will contain fragments of a burst light-source capsule. Accordingly, these and other objects and advantages are achieved by the characterizing features of claim 1. A containment means is mounted within the outer envelope and substantially surrounds the light-source capsule. The containment means comprises a shroud made of a glass which has a strain point greater than the operating temperature of the shroud. The strain point is defined as the temperature at which stress in the glass is substantially removed in about four hours when the glass is maintained at that temperature. The lamp assembly is a metal halide arc lamp, and the light-source capsule comprises a metal halide arc tube. The glass is preferably an aluminosilicate glass which has a strain point of about 600°C or greater. The aluminosilicate glass preferably has a low water content in order to minimize outgassing of water vapor when the lamp is at operating temperature. The shroud preferably comprises an open-ended cylindrical tube, and for lamps with a power rating of about 150 watts or less, the wall thickness is about 1 mm. For a better understanding of the present invention together with other and further objects, advantages, and capabilities thereof, reference is made to the accompanying drawing which is a partially cut-away, side view of a double-enveloped lamp assembly. A metal halide arc discharge lamp 10 is shown in the drawing. A sealed outer envelope 12 (also called an outer jacket) encloses a generally cylindrical shroud 14. The shroud 14 surrounds an arc tube 16 which encloses two electrodes (not shown) and a fill material for supporting an arc discharge. Suitable fill materials are well known to those skilled in the art. Each electrode is coupled through a press seal 36 that hermetically seals the arc tube 16. Electrical energy is coupled from a lamp base 28 through a lamp stem 24 and electrical leads 32 and 34 to the electrodes in the arc tube 16. A mounting assembly 18 mechanically supports the arc tube 16 and the shroud 14. The assembly 18 includes a support rod 20 which is coupled to the stem 24 and to a dimple 22 in the upper end of the envelope 12. The rod 20 is coupled to the shroud 14 with a pair of clips 26. The structure of the lamp is described in more detail in U.S. Patent No. 5,122,706. In that patent, the shroud is fabricated of fused quartz and has a thickness of 2.0 mm or 2.5 mm, depending on the embodiment. The shroud 14 used in the present invention is made of an aluminosilicate glass (Corning 1724 is generally preferred, but other glasses such as GE 177, GE 179, GE 180, Schott 8252, or Schott 8253 can be used.) "Corning" and "Schott" are registered trademarks. "GE" refers to the General Electric Company. The numbers associated with the term "GE" are standard types of glass which are available from General Electric. The term "C 162" refers to a standard published by the American Society for Testing Materials (ASTM) which defines the testing standards for determining the strain point of glass and glass products. The shroud 14 is typically formed as a cylindrical tube open at both ends. In other embodiments, the shroud is closed at one end by a dome shaped portion. The glass shroud preferably has a wall thickness of 2 mm or less, and in a preferred embodiment has a wall thickness of 1 mm (0.039 inches) when it is used in metal halide arc lamps in a range of 75 watts to 150 watts. The glass shroud has the ability to contain fragments of the arc tube, if it bursts, at least as well as a fused quartz shroud with a substantially thicker wall. In the preferred embodiment in metal halide lamps in the above wattage range, the shroud has an inner diameter of 20 mm and an outer diameter of 22 mm, and the length of the shroud ranges from about 46 mm to 56 mm. Other lengths and wall thicknesses can be used in different lamps. The aluminosilicate glass shroud material preferably has a SiO 2 content of 57% to 63%, and an Al 2 O 3 content of 14% to 17%. The remainder can include one or more of the following compounds in differing quantities: B 2 O 3 , MgO, CaO, BaO, SrO, ZrO 2 , Sb 2 O 3 , and P 2 O 5 ; and may include certain impurities. A suitable glass should have a strain point which is greater than the temperature of the shroud when the lamp is operating. The strain point, which is defined by the American Society for Testing of Materials standard terminology of glass and glass products (C162), is the temperature at which a stress, such as tension or compression, can be substantially relieved in a matter of hours. Typically, the shroud temperature is about 500°C during operation. Assuming a 100°C safety margin, the strain point should be at least 600°C. The safety margin is maintained to insure that the strain pattern in the shroud does not change with lamp operating life. The glass should also have high resistivity to maintain an electrical charge on the shroud. The charge inhibits sodium loss from the arc tube. The glass should be dry (have a low water content) so that there is minimal outgassing of water vapor and other gasses, when the lamp is at operating temperature. A measure of the water content is known as beta, which is a measure of the infrared absorption of light in the material due to structurally bonded water. Beta is commonly measured at the absorption peak found at 2.7 - 2.9 micrometers (microns), and is normalized to a 1 mm path length. Consequently, beta values are expressed in mm -1 . A lower value of the beta indicates that a glass is dryer than another glass with a higher beta. The specified measurement wavelength varies for different types of materials, so the beta for an aluminosilicate glass is not directly comparable to the beta for fused quartz. The preferred (Corning 1724) glass has a beta of 0.31 mm -1 . Other types of glass have a lower beta, (e.g., the beta for GE 180 is 0.22 mm -1 .) In general, the value of beta for the glass shroud is preferably less than about 0.4 mm -1 . Referring to Table I below, containment tests were performed on glass and fused quartz shrouds and the results were compared. The test was similar to the Underwriters Laboratories Standard UL1572-57A, which describes containment criteria for a fixture. In these tests, a 100 watt metal halide arc lamp with a (Corning 1724) glass shroud was mounted in a ceramic socket rated for 400 V pulses. A layer of dry, absorbent cotton was positioned about twelve inches below the lamp. The arc tube was exploded by charging a 30 microfarad capacitor connected to the arc tube to between 1500 and 2000 volts. If the cotton was scorched or if it was ignited by the fragments when the arc tube capsule exploded, the result was considered a containment failure. 2.5 mm (0.098") walled Fused Quartz Percentage Quantity Contained, with no outer jacket cracks 65% 88 Contained, with cracked outer jacket 30% 40 Contained, with small hole in outer jacket 5% 7 Did not contain 0% 0 1.0 mm (0.039") walled Fused Quartz Contained, with no outer jacket cracks 23% 12 Contained, with cracked outer jacket 42% 22 Contained, with small hole in outer jacket 8% 4 Did not contain 27% 14 2.0 mm (0.079") Aluminosilicate Glass Contained, with no outer jacket cracks 91% 29 Contained, with cracked outer jacket 9% 3 Contained, with small hole in outer jacket 0% 0 Did not contain 0% 0 1.25 mm (0.049") Aluminosilicate Glass Contained, with no outer jacket cracks 71% 20 Contained, with cracked outer jacket 29% 8 Contained, with small hole in outer jacket 0% 0 Did not contain 0% 0 1.0 mm (0.039") Aluminosilicate Glass Contained, with no outer jacket cracks 71% 52 Contained, with cracked outer jacket 29% 21 Contained, with small hole in outer jacket 0% 0 Did not contain 0% 0 The 2.5 mm fused quartz shrouds contained in every instance, but in a number of cases, a small hole was formed in the envelope. The 1.0 mm fused quartz shroud did not contain in 27% of the cases -- an unacceptable result. A surprising result was that the aluminosilicate glass shrouds with thicknesses of 2.0 mm, 1.25 mm, and 1.0 mm contained in every instance. In no case was even a small hole formed in the outer envelope. Referring to Table II below, fused quartz shrouds and glass (Corning 1724) shrouds were tested and compared for impact resistance. The test shroud was clamped against a wall. A weighted pendulum was raised to a specified angle above the sample and was dropped so that the weight would strike the shroud at about the middle of its length. If the sample did not break, the pendulum was raised by an additional 5° and was dropped again. The angle was increased until the sampie shattered. Note that the angles are relative and are measured by the pendulum device, and are not absolute with respect to a horizontal or vertical plane. As shown in Table II, the 1.0 mm fused quartz shrouds required pendulum angles of 60° and 75° to shatter, while the glass shrouds with the same thickness required pendulum angles of 115° and 85° to shatter. This test demonstrates that the aluminosilicate glass is more impact resistant than the fused quartz of the same thickness. Since the aluminosilicate glass is more impact resistant, less material can be used in the shroud. 1.0 mm (0.039") Fused Quartz Tube Sample #1 required 60° to shatter Sample #2 required 75° to shatter 1.0 mm (0.039") Aluminosilicate Glass Tube Sample #1 required 115° to shatter Sample #2 required 85° to shatter 1.25 mm (0.049") Aluminosilicate Glass Tube Sample #1 required 145° to shatter Sample #2 required 175° to shatter Other types of glass may also be used to fabricate the shroud. For example, a heat treated, dry glass, (such as VYCOR, a registered trademark, available from Corning) can be used. This glass contains about 96% SiO 2 , the remainder including boron, alumina, and alkali. It has a strain point of about 890°C and a beta which is less than 0.4. By using conventionally melted glass instead of fused quartz to manufacture the shroud, the amount of material, the weight, and the cost are all reduced without reducing containment performance. The tests described above show that a glass shroud contains as well as a fused quartz shroud having a wail thickness that is two and one half times greater. The lighter weight of the glass shroud reduces shipping costs and potential damage during shipment and handling. While there have been shown and described what are at present considered the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modification may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims: Use of a light-transmissive glass shroud (14) placed within a sealed light-transmissive outer envelope (12) of a metal halide arc lamp assembly and substantially surrounding a metal halide arc tube (16) for the prevention of the breakage of said outer envelope (12) in the event of an arc tube explosion, wherein said metal halide arc tube (16) is mounted within said outer envelope (12) and wherein said halide arc lamp assembly further comprises means (24,28,32,34) for supplying electrical energy to said arc tube (16), characterised by said light-transmissive glass shroud being of an aluminosilicate glass. Use of a light-transmissive glass shroud according to claim 1, wherein said glass shroud (14) has a strain point temperature greater than the operating temperature of said glass shroud (14) in said lamp assembly (10). Use of a light-transmissive glass shroud according to claim 2, wherein said glass shroud (14) has a low water content so that beta, which is a measure of the infrared absorption of light in the material due to structural bonded water, is less than about 0.4 mm -1 . Use of a light-transmissive glass shroud according to claim 1, wherein said arc tube (16) has a power rating of 150 watts or less, and said glass shroud has a wall thickness of about 1 mm. Use of a light-transmissive glass shroud according to claim 4, wherein said glass shroud (14) comprises an open-ended, cylindrical tube. Use of a light-transmissive glass shroud according to claim 1, wherein said glass shroud (14) is selected to have a strain point temperature of about 600°C or greater.

Inventors: MARLOR RICHARD C, NORTRUP EDWARD H, PARROTT RICHARD A, MARLOR, RICHARD C., NORTRUP, EDWARD H., PARROTT, RICHARD A.
Applicants: OSRAM SYLVANIA INC, OSRAM SYLVANIA INC.
Application year: 1994
Main classification: H01J61
IPC codes: H01J61